A next-generation multimedia wireless communication system, on which research is actively ongoing, is required to process various information such as image data, radio data, and the like, beyond the voice-oriented services at the early stage, and transmit the processed data. Currently, 4th-generation wireless communication system developed following the 3rd-generation wireless communication system aims at supporting a high speed data service of downlink of 1 Gbps (Gigabits per second) and uplink of 500 Mbps (Megabits per second). Downlink refers to communication from a base station (BS) to a user equipment (UE) and uplink refers to communication from the UE to the BS.
The purpose of a wireless communication system is to enable multiple users to perform reliable communication regardless of their locations and mobility. In this respect, however, wireless channels have non-ideal characteristics such as a path loss, noise, a fading phenomenon caused by multi-path, intersymbol interference (ISI), Doppler effect due to terminal mobility, and the like. Thus, various techniques have been developed to overcome the non-ideal characteristics of the wireless channels and enhance reliability of wireless communication.
Reliable techniques supporting a high speed data service may include OFDM (Orthogonal Frequency Division Multiplexing), MIMO (Multiple Input Multiple Output), and the like.
OFDM that can attenuate an inter-symbol interference effect with a low complexity is considered following the post-3rd generation. The OFDM converts serially inputted data symbols into the N number of parallel data symbols, includes them in the N number of separated subcarriers, and transmits the same. The subcarriers maintain orthogonality in a frequency domain. The respective orthogonal channels experience mutually independent frequency selective fading, and the intervals of transmitted symbols are lengthened to minimize the inter-symbol interference. OFDMA refers to a multi-access scheme accomplishing multiple accesses by independently providing portions of available subcarriers to each user in a system using the OFDM as a modulation scheme. The OFDMA provides frequency resources called subcarriers to each user, and in general, the respective frequency resources are independently provided to multiple users so as not to overlap with each other. That is, resultantly, frequency resources are mutually exclusively allocated to the users.
MIMO improves a data transmission and reception efficiency and a spectral efficiency by using multiple transmission antennas and multiple reception antennas. The MIMO technique includes spatial diversity, spatial multiplexing, beamforming, and the like.
A UE matches downlink synchronization through a downlink synchronization channel and matches uplink synchronization through random access or ranging in the process of setting synchronization with a BS. Besides the uplink synchronization, the UE may perform random access to entire a network, perform handover, and acquire uplink radio resources.
When the BS does not have information regarding UE capability such as the number of antennas the UE uses, the BS assumes that the UE uses one antenna and transmits a downlink signal. In this case, although the BS transmits downlink signals through multiple antennas, a transmission method based on diversity is the only method that can be applicable as a multi-antenna scheme. Likewise, in case of the uplink synchronization, the BS cannot know about UE performance and also cannot know about whether UEs accessing the same resource domain have any difference in their performance.
In the random access process, the UE and the BS cannot know about each other, a preamble that can be shared by every UE within a cell is assumed. The UE transmits a preamble through a random access channel (RACH) or a ranging channel to the BS. When the multi-antenna scheme is intended to be applied to the RACH, the RACH should be designed in consideration of basic configuration of the UE. However, the BS cannot previously know about the configuration of the UE. Also, the RACH is a basic channel for every UE within the cell the UE can access without a negotiation with the BS. Thus, in the existing system, the multi-antenna scheme is not applied to the RACH. In the existing system, the process of generating a preamble for a random access is designed on the assumption of a single antenna UE, and preamble is transmitted on the assumption of the single antenna UE.
However, this does not conform with the next-generation wireless communication system. In addition, the use of only a single antenna by a multi-antenna UE degrades power efficiency. In case of a UE using multiple antennas, output power is the sum of power of each antenna. That is, a proper link budget can be maintained between the UE and the BS when the UE transmits a preamble by using all the antennas. Thus, a preamble structure for a random access allowing for an increase in performance according to the number of antennas of the UE is required. In addition, a method for transmitting a preamble that can acquire a diversity gain of the multiple antennas is required. In this case, a preamble must be transmitted without causing interference between UEs having various performances within the cell.
Resultantly, a method for generating a preamble for a random access supporting multiple antennas and a method for performing random accessing by using the generated preamble are required.